نشریه مهندسی دریا
پیاپی 24 (پاییز و زمستان 1395)

The horseshoe vortex generated around the sail-body junction of a submarine has an important influence on the uniformity of the submarine wake at the propeller and hydrodynamic noise. The influence of the horseshoe vorex on the submarine performance is not negligible. A suitable way to reduce the effects of this separated flow is to use vortex generators. The main goal of the present study is to investigate the flow field around a standard underwater model employing the vortex generator by using the oil flow visualization method and CFD method (OpenFOAM code) in 0°≤α≤30° angles of attack. In this study, the application of experimental method in wind tunnel and CFD simulation which can help us to precisely study the structure of vortical flow field. The results show that Vortex Generators placed along the submarine do indeed significantly reduce strength of the horseshoe vortex, line separation, size of cross-flow vortices and drag force.

Aggressive marine environments and tidal conditions due to chloride ions attack and subsequent wetting and drying concrete, lead to excessive corrosion of reinforcing steel that creates destruction. Simulating behavior of corrosion can be effective in reducing the damage, regional investigations are necessary for durability-based design and evaluation of the models proposed for service-life prediction. In this paper Monte Carlo simulation use for service-life prediction of reinforced concrete structures in tidal zone. This study evaluates the influence of mean and standard deviation variations for each of the parameters that affect on the time of corrosion initiation. In tidal condition that studied in this paper, in the case of 10 mm convection depth with respect to non tidal zone, the service life of the structures decrease about 15 years. So it is found the effect of corrosion occurrence to standard deviation of concrete cover that must be noticed in constructions of marine structures.

In this paper the problem of coordinated path following for a group of Autonomous Underwater Vehicle (AUV) subjected to obstacle and collision avoidance is considered. At first a back stepping controller is used for an AUV to design a path following controller and its stability is examined via Lyapunov criteria. Then using of graph theory, modeling of interconnection between AUV systems is addressed and by this modeling the coordinated path following controller for multiple AUV is designed. After all the problem of obstacles and collision avoidance is considered appropriately. For this purpose, each obstacle involving a limit cycle of an ellipse and if the collision or conflict of one AUV to the surrounded environmental obstacles will be detected, the obstacle avoidance algorithm is activated to prevent collision. Also the same algorithm for collision avoidance in between all AUV systems in the group is implemented. Finally the depicted results illustrate the effectiveness and performances of the proposed controller apparently for a sample AUV,which its hydrodynamics coefficients are available in references.

Stopping the production, heavy loss and environmental hazards are the outcomes of damaging the offshore pipelines. Several cases of damaging because of fishing activities and clash anchors are reported in the Persian Gulf annually. In this paper, response of pipeline against the traction of hooking the anchor and trawling the gear is studied. In other words, the strength of a pipeline against a lateral load will be evaluated. The actual situation of a gas pipeline lying on the seabed of the Persian Gulf is modeled regarding soil, reinforced concrete and non-linear parameters. The load is considered statically which occurred laterally. This research is performed by the finite element method and will be compared to an experimental test for validation. The results show that a 4-inch pipe has low strength against the lateral loads.

In this study, Mazandaran province's coasts have been classified by using Relative Tide Range, and hydrodynamic models. For this, investigations of waves hydrodynamics, measurement of bed morphology's profiles and sediment characteristics along surfzone, and analyzing of hydrographic maps at 7 stations in east (Neka, Larim and Babolsar), middle (Mahmudabad and Kohnesara) and west parts (Nushahr and Nashtarud) of Mazandaran province have been done. results show that Mazandaran coasts are wave dominate that, by wave energy decreasing and wave period and sediment diameter increasing from east to west of Mazandaran, beach states have been changed from Dissipative to Intermediate and Reflective states, respectively. Seasonal investigations of wind power and morphodynamical structures show that variations in bed morphology and wave dynamics do not cause beach state's changes, seasonally.

Existence of coastal regions in Iran has made this country a potential area to produce hydro-electric. Ducted or shrouded turbines generate more output power by increasing fluid velocity; therefore, they turn potential areas to actual areas to produce electricity. The purpose of this study is to investigate effects of a shroud with airfoil cross-section form on upwind and downwind streams which is a vital parameter of performance and usage of this kind of turbines. To do this, the fluid velocity was measured at eight sections behind and in front of the shroud in total and the velocity profiles were compared with numerical simulation results. The results showed that the inconsiderable increase of pressure in front and fast disappearance of the effects of shroud behind it, which is as a result of specific streamed-line design, are some advantages that cause to install more shrouds in a defined area.

Launching pads are among the essential elements of missiles and their main function is to prepare missiles for launch. Tower launchers dimension and configuration depend on effective loads, dimension and weight of the missile. In the majority of tower launchers, two bars connected to each other with crossbar connectors are considered to be the basic parts. Therefore, it is essential to calculate accelerations (forces) applied from ship to structure of the tower launcher. Space frame element is used for designing and a finite element code has been written in MATLAB Software to calculate stress and deformities of the structure. A Genetic algorithm which is codified in MATLAB has been used for optimization. All of the written codes have been compared to experimental data and results confirm the accuracy and exactness of the outcome. In this design weight of the tower launcher was the target equation, displacement and yield stress were constraints and geometrical specifications of the cross section were variables.

In this paper the effect of tail form on maneuverability of autonomous underwater vehicle is investigated. In the beginning, tail form is defined using a mathematical function dependent on two parameters including length of the tail and its cone angle. Then the effect of these two parameters on the hydrodynamic coefficients and maneuverability is investigated respectively. Hydrodynamic damping coefficients are calculated by applying the numerical simulation of oblique towing and rotating arm tests using computational fluid dynamics (CFD) with CFX solver and added mass coefficients are obtained using strip theory method. Due to the different nature of coefficients, using a combination of both these methods has improved the accuracy and speed of calculations. Results show that hydrodynamic damping and added mass coefficients and the turning radius decrease with increasing the tail length, however maneuverability increases. On the contrary, the results are obtained for tail cone angle.

The Caspian Sea level changes were very high in various periods of time; alternatively decreasing and increasing. Since the Caspian Sea level changes generally are remarkable in the short period of time, shoreline forward and retreat will be large due to the fluctuation of sea level; so, the prediction of shoreline changes is very important in the Caspian Sea. Bruun has presented his theory under the title of Bruun rule for shoreline change prediction; this rule is related to sea level rise, closure depth, distance from the coastline to closure depth and the maximum vertical run-up excursion above the point of bore collapse parameters. Also, Rosati et al. applied landward transport to the Bruun rule for improvement of its prediction that achieved the most favorable results.
In this research, the Bruun rule and modified Bruun rule extended for landward transport has been investigated for the Caspian Sea. In this study, the modified equation has shown the better results, which suggests that landward sedimentation could be the result of the Sea level changes as well as natural storms during the years.

The major problems in modeling of different oceanographic and meteorological parameters are limitations in numerical methods and human incomplete knowledge in physical processes involved. As a result, significant differences between the results of these models and in situ observations of these parameters might exist. One of the powerful solutions for decreasing the forecast errors in the models is to use data assimilation technique. In this study the optimal interpolation data assimilation method is employed which is based on statistical rules. Moreover, the quick Canadian method is used to optimize the data assimilation method used in model. Model assessment is performed by comparison between running wave model with and without using data assimilation and SAR wave data in Persian Gulf. It shows that using data assimilation in WAVEWATCH-III model reduces the error in wave height predictions significantly.

This paper presents a special underwater robot for subsea pipelines inspection which is used to transport extracted oil and gas from oil platforms to onshore facilities. Due to the high pressure in the deep sea and the long pipelines, it is impossible to inspection by divers. Therefore underwater robot can be used to solve this problem. This paper investigate a hovering type of underwater vehicle including four thruster with variable thrust vector, which it is able to simultaneous control of six degrees of freedom. Therefore, high flexibility is an advantage of this underwater robot. Using two perpendicular servomotors for each thruster, we prepare independent and time variable orientation for each thruster. This paper investigates a trajectory controller for this hovering type autonomous underwater vehicle to meet the demands of in-water pipeline inspection.